A conventional flow amount measuring device for measuring a flow amount of fluid injected by a fluid injection valve is disclosed in Japanese Examined Utility Model Publication S51-38777. The flow amount measuring device therein utilizes a gear pump installed in a flow path of the fluid and a piston moving under a pressure difference of the fluid between an upstream side and a downstream side of the gear pump. The flow amount measuring device detects the location of the piston and rotates the gear pump by means of a motor so that the piston returns to a reference position. When the flow amount increases, the pressure difference becomes larger and the pump gear is rotated faster to bring back the piston to the reference position. Therefore, the flow amount measuring device can detect the flow amount according to the amount of rotation of the gear pump.
However, the rotation of the gear pump causes a pressure pulsation in the fluid which makes the position of the piston unstable. Therefore, the flow amount measuring device utilizing the rotation of the gear pump has difficulty in measuring the flow amount with a high degree of accuracy.
In addition, although the flow amount measuring device utilizing the rotation of the gear pump may be suitable for measuring a flow amount per unit time of fluid flowing continuously in the flow path, it has difficulty in measuring with a high degree of accuracy the flow amount of fluid injected by the fluid injection valve repeating the injection intermittently, such as a fuel injection valve for an internal combustion engine.
In addition, when the fluid injection valve starts the injection for the purpose of measuring the flow amount, there is a delay from a time, at which the pressure difference between the upstream and downstream side of the gear pump is detected, to a time at which the rotation of the gear pump driven by the motor reaches a rotational speed for canceling the pressure difference. Therefore, the amount of the rotation detected in the delay period does not represent a correct flow amount. In order to cope with the problem caused by the delay period, the measuring device may neglect the detected amount of rotation in the delay period, and uses the detected amount of rotation after a predetermined time period has passed by after the injection valve had started its injection. Otherwise, the number of injections, including the injections during the delay period, is increased to diminish the influence of the incorrectness caused by the delay time.